Problem: The scale on a map is 3cm : 8km. If the distance between two cities is 72km, how far apart in cm are the two cities on the map?
Solution: The scale means that every 3cm on the map corresponds to an actual distance of 8km. An actual distance of 72km is the same as 9 $\cdot$ 8km. The distance between the two cities on the map is 9 $\cdot$ 3cm, or 27cm.